1. Field of the Invention
The present invention relates to a tempo detection apparatus and a tempo-detection computer program.
2. Discussion of Background
A tempo detection apparatus has been developed for detecting beat positions from a musical acoustic signal (audio signal) in which the sounds of a plurality of musical instruments are mixed, such as the audio signals of music compact discs (CDs).
In that apparatus, to detect beat positions, a fast Fourier transform (FFT) is applied to an input waveform at predetermined time intervals (frames); the power of each note in a scale is obtained from the obtained power spectrum; an incremental value of the power of each note in the scale at each frame interval is calculated; the incremental values are summed up for all the notes in the scale to obtain the degree of change of all the notes at each frame interval; the autocorrelation of the degree of change of all the notes at each frame interval is calculated to obtain periodicity; and an average beat interval (so-called tempo) is obtained from the frame interval which maximizes the autocorrelation.
When the average beat interval is obtained, the degrees of changes of all the notes at frames separated by beat intervals are added up with the starting frame being shifted by one frame, in frames (having a length about ten times the average beat interval, for example) at the top portion of the waveform, and the starting frame which maximizes the total value is regarded as the starting beat position.